Actually, from a technical viewpoint, the CPU socket itself is an expansionbus. All upgrades for ST(E) in one way or another hooks into the very same pins the CPU sits on, although some do it along the path to another chip.
On Mega ST, the expansionbus basically was all the CPU pins put in a socket.
On Mega STE/TT, the bus is more limited, but many pins from the CPU found it's way to the VME Bus.
Since all upgrades not intended for Mega or TT machines requires soldering, an expansionbus by itself is pointless since there is no real standard for it, and also, most upgrades in a sense incorporates their own expansionbus.
To make things a bit more clear, the expansionbus I talk about in my project is just an idea. But for it to be meaningful, I would need someone to actually be interested in using it, and got a good idea of what he needs from me.
However, to utterly confuse things, one expansionbus that indeed could be useful, is an ISA port. The only one I'm aware of for ST(E) is using the cartridge port. However, anyone intending to use an ISA card with their ST(E) will have to write their own drivers.
So start saving up on ISA cards.